codify "platform" into resolution logic, server-fn/fullstack overhaul (#4596)

overhaul dioxus fullstack using a new implementation of server_fn

This PR is a complete rewrite of dioxus fullstack designed to improve compatibility with axum and to fix a number of compilation errors based on feature flag unification plumbing through macros.

packages/fullstack/src/spawn.rs

@@ -0,0 +1,31 @@
+use std::future::Future;
+
+/// Spawn a task in the background. If wasm is enabled, this will use the single threaded tokio runtime
+pub(crate) fn spawn_platform<Fut>(
+    f: impl FnOnce() -> Fut + Send + 'static,
+) -> tokio::task::JoinHandle<Fut::Output>
+where
+    Fut: Future + 'static,
+    Fut::Output: Send + 'static,
+{
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        use tokio_util::task::LocalPoolHandle;
+        static TASK_POOL: std::sync::OnceLock<LocalPoolHandle> = std::sync::OnceLock::new();
+
+        let pool = TASK_POOL.get_or_init(|| {
+            LocalPoolHandle::new(
+                std::thread::available_parallelism()
+                    .map(usize::from)
+                    .unwrap_or(1),
+            )
+        });
+
+        pool.spawn_pinned(f)
+    }
+
+    #[cfg(target_arch = "wasm32")]
+    {
+        tokio::task::spawn_local(f())
+    }
+}